Exposure control for an imaging system

ABSTRACT

A device that analyzes an image. The device includes a circuit that receives an image that includes a plurality of pixels. The circuit creates a histogram of the image and analyzes the histogram to determine an acceptable exposure of the image. The histogram may include a plurality of bins versus a population of pixels associated with each bin. By way of example, the bins may be associated with an intensity of light. The images and histograms may include data defined by low dynamic range number of bits and/or an extended dynamic range number of bits. Certain features and criteria of the image may be determined and analyzed to determine whether the image has an acceptable exposure. If the image is unacceptable, an exposure characteristic can be changed and the process can be repeated until an acceptable image is obtained.

REFERENCE TO CROSS-RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/218,742, filed on Jul. 16, 2008, which claims priority to ApplicationNo. 60/962,028 filed on Jul. 25, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject matter disclosed, generally relates to automatic exposurecontrol in a digital camera.

2. Background Information

Digital cameras contain an image sensor that converts optical energyinto electrical signals. Most digital cameras are equipped withautomatic exposure control. Exposure control involves determining theexposure settings that best utilize the dynamic range of the imagesensor given the characteristics of an image scene being photographed.The dynamic range of an image sensor may be defined as the ratio of thehighest signal output to the lowest useful signal output.

Image sensors have a limited dynamic range. A typical electronic imagesensor has a dynamic range less than the dynamic range of naturalscenes. This is mainly due to multiple light sources with widely varyingintensities illuminating the scene objects.

Generally speaking it is desirable to have an exposure so that thecaptured image is not too bright or too dark. Commonly, this is achievedthrough moving an image histogram to an optimal point within the boundsof maximum and minimum output signal levels of the system. Thehistograms are frequency distributions of image pixels based onintensity values.

Exposure control in a camera typically involves measuring the averageintensity of light from a sample area of an image scene beingphotographed. Exposure settings are then selected to scale the averageto a pre-determined percentage of the dynamic range of the image sensor.One technique, commonly referred to as mean photometry, includesdetecting the entire image region and controlling the exposure so thatthe detection signal has a constant level. Another method, commonlyreferred to as center-emphasizing photometry, detects only a centralportion of the image region and controls the exposure so the detectionsignal has a constant level. A combination of mean photometry andcenter-emphasizing photometry can be accomplished by weighting thedetection data of the entire video region and the detection data of thecenter region, and performing exposure control based upon detection dataobtained by adding the weighted data at a fixed ratio. A finer exposurecontrol can be achieved by sub-dividing a scene into areas, detectingvideo in each area and limiting the areas of detection data used inexposure control or changing the weighting. However, even thephotometric methods described above do not always provide a state ofexposure control that is suitable for the scene in question.

BRIEF SUMMARY OF THE INVENTION

A device that analyzes an image. The device includes a circuit thatreceives an image that includes a plurality of pixels. The circuitcreates a histogram of the image and analyzes the histogram to determinean acceptable exposure of the image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing a block diagram of an image pickupapparatus;

FIG. 2 is an illustration showing components of the imaging system;

FIG. 3 is a graph showing an example of an image histogram;

FIG. 4 is an illustration showing components of a first embodiment of ahistogram generation unit;

FIG. 5 is a flowchart describing the process of the first embodiment ofthe histogram generation unit;

FIG. 6 are pictorial descriptions of three LDR images of differentexposures composed together to form one EDR image;

FIG. 7 is an illustration of a process for the composition of an EDRimage from multiple LDR images;

FIG. 8 are pictorial descriptions of the extraction of three LDR imagesof different exposures from an EDR image;

FIG. 9 shows a flowchart describing the generation an LDR image from anEDR image;

FIG. 10 are pictorial descriptions of the generation of histograms fromfour different channels: Red, Green, Blue and Luminance from one LDRimage;

FIG. 11 is an illustration of a second embodiment of the histogramgeneration unit;

FIG. 12 shows a flowchart describing a process of the second embodimentof the histogram generation unit;

FIG. 13 are pictorial descriptions of the generation of EDR histogramsfrom four different channels: Red, Green, Blue and Luminance from oneEDR image;

FIG. 14 includes a number of graphs depicting the generation of threeLDR histograms from an EDR histogram;

FIG. 15 is a flowchart showing the extraction of LDR histograms from oneEDR histogram;

FIG. 16 includes a number of graphs depicting the generation of fourdifferent LDR histograms, each with a different exposure from an EDRhistogram;

FIG. 17 is an illustration for a embodiment of histogram generationunit;

FIG. 18 is a flowchart illustrating the steps for the third embodimentof histogram generation unit;

FIG. 19 illustrate four LDR images with corresponding red channelhistograms;

FIG. 20 is a flowchart illustrating the steps for a fourth embodiment ofhistogram generation unit;

FIG. 21 is a flowchart illustrating the steps for a fifth embodiment ofhistogram generation unit;

FIG. 22 is a flowchart illustrating the steps for a sixth embodiment ofhistogram generation unit;

FIG. 23 is a flowchart illustrating the steps for a seventh embodimentof histogram generation unit;

FIG. 24 is an illustration of an embodiment of a features extractionunit;

FIGS. 25 a-c are graphs showing three types of histogram peaks;

FIG. 26 is a flowchart illustrating the steps for a sky-peak detectionunit;

FIGS. 27 a-b are graphs showing an example of similar rightmost peaks inthe histograms of the green and blue channels;

FIGS. 28 a-b are graphs showing an example of the last vertex of theblue histogram being significantly higher than the red histogram;

FIGS. 29 a-c are graphs showing an example of the summit of therightmost peak of the blue histogram higher than both the red and greenhistograms;

FIGS. 30 a-c are graphs showing an example of the last vertex of theblue histogram taller than the last vertexes of the red and greenhistograms;

FIG. 31 is a flowchart explaining the measurement of Significant Peakparameter;

FIG. 32 are graphs that show a pictorial description of the measurementof the Significant Peak parameter;

FIG. 33 is a flowchart describing the steps for the measurement of aRight Peak parameter;

FIG. 34 are graphs that show a pictorial depiction of the measurement ofthe Right Peak parameter

FIG. 35 is an illustration of an optimum exposure determination unit;

FIG. 36 is a flowchart illustrating a process performed by the optimumexposure determination unit;

FIGS. 37 a-b are graphs that show a first example of a Dark Detailmeasurement;

FIGS. 38 a-b are graphs that show a second example of a Dark Detailmeasurement;

FIG. 39 is a flowchart detailing the measurement of Sky Contrast;

FIGS. 40 a-b are graphs that show an example of a Sky Contrastmeasurement;

FIG. 41 is a flowchart detailing the measurement of a Contrast Peak;

FIGS. 42 a-b are graphs that show a first example of a Contrast Peakmeasurement;

FIGS. 43 a-b are graphs that show a second example of a Contrast Peakmeasurement;

FIG. 44 is a flowchart detailing the measurement of Fluct;

FIGS. 45 a-d are graphs that show an example of a Fluct measurement;

FIG. 46 is a flowchart showing a process to increase exposure;

FIG. 47 is a flowchart showing a process to decrease exposure.

DETAILED DESCRIPTION

Disclosed is a device that analyzes an image. The device includes acircuit that receives an image that includes a plurality of pixels. Thecircuit creates a histogram of the image and analyzes the histogram todetermine an acceptable exposure of the image. The histogram may includea plurality of bins versus a population of pixels associated with eachbin. By way of example, the bins may be associated with an intensity oflight. The images and histograms may include data defined by a lowdynamic range number of bits and/or an extended dynamic range number ofbits. Certain features and criteria of the image may be determined andanalyzed to determine whether the image has an acceptable exposure. Ifthe image is unacceptable, an exposure characteristic can be changed andthe process can be repeated until an acceptable image is obtained.

Referring to the drawings more particularly by reference numbers, FIG. 1shows an embodiment of an image pickup apparatus 102. The embodimentrepresents a digital still camera, but there can be many differentembodiments that can make use of the imaging system to control exposureof an image. The image pickup apparatus 102 includes a lens 104, anaperture 106, an image sensor 108, an A/D converter 110, a processor112, a display 114 and a memory card 116. Light from a scene entersthrough the lens 104, while the aperture 106 controls the amount oflight entering into the image sensor 108. The analog signal obtainedfrom the image sensor 108 is converted to a digital signal by the A/Dconverter 110. The digital signal is then sent to the processor 112 forvarious processes, such as interpolation. An exposure control value willbe generated by the processor 112 and this value will alter the settingsof either the aperture 106 and/or the time integration device locatedinside the image sensor 108. The final image deemed to be suitable foroutput will either be displayed onto the display 114 or stored in thememory Card 116. The processor 112 performs the imaging processes.

Exposure is defined as the amount of light entering the image pickupapparatus 102. It can be calculated from the size of the aperture 106given in terms of F-number, e.g. F4.5 and the shutter speed of the imagepickup apparatus 102, given in terms of exposure time, e.g. 1/125 s. Theexposure value can be calculated using the following formula:

${{EV} \propto \frac{t}{N^{2}}},$

where EV is the exposure value, N is the F-number and t is the exposuretime.

FIG. 2 is a block diagram of an embodiment of an image system 204 withinputs and outputs. The imaging system 204 can be implemented as a partof the electronic device, for example as a processing unit in a digitalcamera or similar, such as the one described above. An image pickupapparatus can be a digital still camera but is not limited to such anembodiment. It can be a scanner, a digital photo kiosk, a computer withdigital images or any device that can provide digital images to theimaging system 204 and memory to run the imaging system. An imageproviding unit 202 provides a digital image for the imaging system 204.An image providing unit can be a memory card, a hard-disk with digitalimages, or any device that can provide the imaging system 204 withdigital images.

The imaging system 204 includes a histogram generation unit 206, ahistogram features extraction unit 208, (or features extraction unit inshort) and an optimum exposure determination unit 210 that provides anoptimum exposure image 212. The histogram generation unit 206 generateshistograms from the digital images provided by the image providing unit202. The generated histograms are then sent to the features extractionunit 208, where various features of the histogram are measured andcalculated. These features are input to the optimum exposuredetermination unit 210, where adequacy of exposure is measured based onthese features. As shown by the arrow 214 the imaging system 204operates in a loop fashion, where units 206, 208 and 210 are repeateduntil an image with an optimum exposure 212 is obtained. The optimumexposure determination unit 210 calculates and outputs a tentativeexposure value, Tx to the Histogram Generation Unit 206 for each loopiteration.

FIG. 3 shows an example of a histogram generated from an image. An imagehistogram is a graphical representation of the number of pixels in animage as a function of intensity. The intensity of a pixel can beexpressed as a digital number measuring a certain color, hue, brightnessor other dimensions of light captured in the image. Histograms consistof vertices 302 which are points on the histogram. The x-axis 304 ismade up of bins, each bin representing a certain intensity value range.The histogram is computed by examining all pixels and assigning each toa bin depending on the pixel intensity. The y-axis 306 represents thepopulation of pixels of the image belonging to each bin. For example, a256-bin red-channel histogram for an 8-bit RGB image with red channelvalues ranging from 0 to 255 has bin 1 representing red values of 0; bin2 of 1 and so on, while a 16-bin histogram has bin 1 representing redchannel values from 0 to 15; bin 2 from 16 to 31 and so on.

The dynamic range of an image refers to the ratio of the largestpossible signal value, divided by the smallest non-zero signal value.For example, in an 8-bit RGB digital image, the largest signal value is255 and the smallest non-zero signal value is 1. The dynamic range of an8-bit image is 255. A low dynamic range (LDR) image typically refers toan 8-bit image with a dynamic range of 255, whereas an extended dynamicrange (EDR) image refers to an image with a dynamic range greater than255.

Referring to FIG. 2, the Histogram Generation Unit 206 receives digitalLDR images and outputs a set of LDR histograms of a tentative LDR image.There can be a variety of embodiments in which this can be carried out,and a few of them are specified here. The feature extraction unit 208receives the LDR histograms and calculates parameters of the histogramsthat can indicate significant features of the tentative image. Thetentative image may or may not be generated by the imaging system 204,depending on the embodiment of histogram generation unit 206. Theoptimum exposure determination unit 210 within the imaging system 204checks for adequate exposure of the tentative image by examiningfeatures it receives from the feature extraction unit 208. If theexposure is determined to be unsuitable for the scene, the optimumexposure determination unit 210 calculates a new tentative exposurevalue Tx for the next tentative image and provides the histogramgeneration unit 206 with Tx to produce a new tentative image withexposure Tx. A new tentative image of a different exposure is processedto generate a new histogram. The features are again extracted and thereis a determination of exposure adequacy. This process is repeated untila tentative image is deemed to have suitable exposure and is provided asan output 212.

FIG. 4 is a block diagram of an embodiment of the histogram generationunit 206. The unit receives a plurality of LDR images of differentexposures 402 that are processed by an EDR image generation unit 404, animage generation unit 406 and a histograms calculation unit 408. Thehistogram generation unit 206 provides LDR histograms 410. Also, atentative exposure value, Tx 412 is provided to the image generationunit 406.

FIG. 5 is a flowchart describing a process for generating histograms. Aplurality of LDR images of different exposures are obtained in block 502either using a exposure bracketing mode or any other method to obtainimages of the same scene with different exposures. The EDR image isgenerated from the LDR images in block 504. An EDR image reduces the useof memory to store many LDR images of different exposures. This isbecause an LDR image of an exposure within a predefined range can beextracted from the EDR image, which has a range determined by theminimum exposure value, Tmin, and the maximum exposure value, Tmax, ofthe LDR images. In block 506 a single LDR image of a tentative exposureTx is then generated from the EDR image. The Tx falls within thepredefined range. LDR histograms of the image are generated from thesingle LDR image in block 508.

FIG. 6 shows an example of an EDR image formed from three LDR images ofdifferent exposures. Three LDR images with increasing exposures areshown on the left hand side of FIG. 6, including LDR 1 602 with thelowest exposure, T1 (darkest), LDR 2 604 with exposure, T2 and LDR 3 606with the highest exposure, T3 (brightest). Together, the LDR images areused to compose an EDR image 608 shown on the right hand side of FIG. 6.Each LDR image is an 8-bit RGB image in this example; hence the EDRimage created is a 10-bit RGB image. A different number of LDR imagesmay be used to compose an EDR image, as long as the number exceeds one.Hence the description is not meant to limit the method to compose an EDRimage, but as a simplified explanation to help grasp the concept.

FIG. 7 illustrates a process for creating an EDR image 608 from thethree LDR images 602, 604, and 606. The brightest of the three shots,LDR 3 606 of exposure T3 provides the first input to the system. The LDRimages may contain nonlinear distortions created during the imageacquisition process. One example of nonlinear distortion occurs in theimage pickup device wherein the analog circuit therein may have anamplifying gain that varies with signal level. A second example is thesensing node of the image pickup device has a capacitance value thatvaries with signal level. A third example is where LDR pictures receivegamma pre-distortion commonly found when producing digital images forcomputer displays. LDR 3 606 is processed by a linearizing unit 702,which applies a linearizing function on the intensity values of eachpixel to remove the nonlinear distortions. The linearizing functiontransforms the images from non-linear to linear images. An example of agamma function and a corresponding inverse gamma function is shownbelow. The gamma function:

${y = {255\; \frac{\left( {1 - ^{{- \frac{x}{255}} \times G}} \right)}{\left( {1 - ^{- G}} \right)}}};$

For an 8-bit image

The inverse gamma function:

${x = {{- \frac{255}{G}}{\log \left( {1 - {\frac{y}{255}\left( {1 - ^{- G}} \right)}} \right)}}};$

For an 8-bit image.

The intensity values of the linearized LDR 3 are then mapped to a lowerexposure value T2 by mapping unit 704. T2 is the exposure value of thedarker image LDR 2 604. This is done by multiplying the ratio of T2 toT3 (i.e. T2/T3) since the intensity values of the linearized LDR 3 606are already made linear by the Linearizing Unit 702. The resultant imageis processed by a de-linearizing unit 706. The output of unit 706 iscombined with the intensity values of LDR 2 604 by a combining unit 1708.

The combination is done by replacing “brighter pixels” in the output ofthe de-linearizing unit 706 with the LDR 2 604 pixels to form one outputimage. A “brighter pixel” in the output of de-linearizing unit 706 is apixel where at least one color channel has an intensity value thatexceeds a first predetermined threshold, e.g. 127 for intensity valuesbetween 0 and 255. Combining Unit 2 710 replaces the “dimmer pixels” ofLDR 2 604 with pixels in the output of Combining Unit 1 708. A “dimmerpixel” in LDR 2 604 is a pixel where at least one color channel has anintensity value below a second predetermined threshold, the secondpredetermined threshold may or may not be identical to the firstpredetermined threshold. The combined operation of combining unit 1 708and combining unit 2 710 is equivalent to replacing a “dimmer pixel” inLDR 2 with the output of the de-linearizing unit 706, if and only if thelatter is not a “brighter pixel”.

This operation avoids a kind of image artifact described below. If ascene has brightness that gradually changes monotonically across theframe, the LDR 2 604 pixels are replaced with the output of thede-linearizing unit 706 where the pixels become dimmer. However, due tosome inaccuracy in linearizing or de-linearizing, the output from thede-linearizing unit 706 is slightly brighter than it is supposed to be.What shows up in the resultant image is an edge where the brightness inthe scene crosses over the threshold. Combining Unit 708 checks to makesure that the output pixels of the de-linearizing unit 706 is notbrighter; otherwise LDR 2 pixels will be kept.

The output image of unit 710 is processed by the same process as LDR 3606 above except that it is now re-mapped to an even lower exposurevalue T1 by mapping unit 712, T1 being the exposure value of the darkestimage LDR 1 602. The second input image to the two combining units 708and 710 is changed to LDR 1 602. The output image at the secondcombining unit 2 710 is then processed by linearizing unit 702. Theresultant image is multiplied with a multiplier N, by a multiplicationunit 714. N is the ratio of the maximum exposure, T_(max) to the minimumexposure value, Tmin of the plurality if LDR images (i.e. Tmax/Tmin).The ten-bit EDR image 608 is produced by unit 714.

Referring to FIG. 4, after an EDR image is generated from the EDR imagegeneration unit 404, a tentative LDR image with a tentative exposure isgenerated from image generation unit 406. The tentative exposure valueof the LDR image is between the minimum and maximum exposure values ofthe plurality of LDR images 402 used to create the EDR image. Forexample as shown in FIG. 8, three LDR images 802, 804 and 806 areextracted from an EDR image 808. These three images have exposures withvalues between Tmin and Tmax, which are the minimum exposure value andmaximum exposure value, respectively.

FIG. 9 illustrates how an LDR image 806 is generated from an EDR image808. First the intensity values of the EDR image 808 are processed bythe multiplication unit 714 similar to that shown in FIG. 7. Theintensity values of the EDR image 808 are multiplied with a multiplier1/A at 714, where A is the ratio of the maximum exposure to thetentative exposure value for the LDR 3 image 806. The multiplied EDRimage is processed by the de-linearizing unit 706, and a clamping unit902, which clamps the intensity values of the image such that the imagebecomes an LDR image. For example if a system defines an LDR image asany 8-bit image, the system will clamp the intensity values ofde-linearized EDR image 806 to a maximum value of 255. The LDR 3 806 isthe output of the clamping unit 902.

Referring to FIG. 4, the Histogram Calculation Unit 408 receives thetentative LDR image created by the image generation unit 406 andcalculates tentative LDR histograms based on the tentative LDR image anda tentative exposure value Tx. FIG. 10 shows the calculation of four LDRhistograms 1004, 1006, 1008 and 1010 from an LDR image 1002 for fourdifferent intensity channels; namely, a red channel 1004, a greenchannel 1006, a blue channel 1008 and a luminance channel 1010. Othercolor space histograms such as HSV histograms or CMYK histograms canalso be calculated, but are not shown in the drawings.

FIG. 11 shows another embodiment of a histogram generation unit 206. Thehistogram generation unit 206 creates EDR histograms from an EDR imagethen further creates tentative histograms from the EDR histograms,instead of creating a tentative LDR image. The unit 206 then creates aset of tentative LDR histograms as in the previous embodiment. Similarto the previous embodiment, LDR images of different exposures 402 areused to create an EDR image in EDR image generation unit 404. The EDRimage is then used to create corresponding EDR histograms based on thepixel intensity values of the EDR image by an EDR histograms generationunit 1102. LDR histograms 410 are then calculated from the EDRhistograms in the histograms calculation unit 1104 using the tentativeexposure, Tx 412.

A histogram is usually generated from an image of a certain exposurevalue. In this embodiment, LDR histograms can be generated directly froman EDR histogram without generating LDR images. An LDR histogramgenerated from an EDR histogram at a particular exposure valuecorresponds to the LDR histogram generated from an LDR image of the sameexposure value. Hence, an LDR histogram having a specific exposure valueis defined as the same LDR histogram that will be generated from animage of that specific exposure value.

FIG. 12 is a flowchart for a process to generate an LDR histogram. Aplurality of LDR images of different exposures is first obtained inblock 1202. The LDR images are used to generate an EDR image in block1204. In block 1206 EDR histograms are then calculated from the EDRimage. Finally, a tentative exposure, Tx is used to generate tentativeLDR histograms from the EDR histograms in block 1208.

FIG. 13 is an illustration showing the creation of EDR histograms. AnEDR histogram is calculated in the same way that an LDR histogram iscalculated from an LDR image. The population of pixels in the EDR imagebelonging to each bin is counted and this becomes the y-value of the EDRhistogram for each bin. On the left-hand side of FIG. 13, an example ofan EDR image 1302 is shown. Using the pixel intensity values of the EDRimage 1302, four EDR histograms 1304, 1306, 1308 and 1310 are generated.The four histograms corresponding to the four intensity channels: red,green, blue and luminance (brightness) respectively are shown on theright hand side of FIG. 13.

FIG. 14 shows a EDR histogram 1402 and a plurality of LDR histograms1404, 1406 and 1408. For a 10-bit EDR image which was composed from8-bit LDR images, the EDR histogram 1402 generated from the EDR imagehas bin values extending from 0 to 1023. The LDR histograms 1404, 1406,and 1408 extracted from the EDR histogram will have bin values from 0 to255. Extraction of LDR histograms from an EDR histogram is similar tohow LDR images are extracted from an EDR image.

FIG. 15 shows an embodiment for extracting an LDR histogram 1404 from anEDR histogram 1402. Similar to FIG. 9, FIG. 15 shows the EDR histogram1402 being processed by a multiplication unit 714, a de-linearizing unit706 and a clamping unit 902 to create the LDR histogram 1 1404. The binvalues of the EDR histogram 1402 is first processed by themultiplication unit 714 where values are multiplied with a multiplier1/A, where A is the ratio of the maximum exposure to the exposure valueof the LDR histogram 1 1404. Thereafter, the multiplied EDR histogram isthen processed by the de-linearizing unit 706, and a clamping unit 902,which clamps the bin values of the resultant histogram to a maximumvalue of 255.

Similar to the concept of an EDR image being composed from multiple LDRimages, an EDR histogram can also be composed from multiple LDRhistograms. Hence, this process is also reversible, such that, LDRhistograms can be extracted from an EDR histogram. Two different bins ofan EDR histogram can map to the same bin of the LDR histogram, in whichcase the populations of both bins of the EDR histogram are summed toproduce the population of the LDR histogram's bin. In general,populations of all EDR histogram bins that map to the same LDR histogrambin are summed together to produce a population for the latter.

FIG. 16 provides an illustration where four different exposure values,T1, T2, T3 and T4 are used to create LDR Histogram 1 1604, LDR Histogram2 1606, LDR Histogram 3 1608 and LDR Histogram 4 1610 from an EDRhistogram 1602, respectively. Each LDR histogram represents the LDRhistogram that will be calculated from the pixel intensity of an LDRimage at that same exposure value.

As compared to the previous embodiment, this embodiment has theadvantage of using less memory space of a processor and reduces theprocessing time as well. This is due to the memory used to store ahistogram is significantly less than that of its corresponding image.Furthermore, calculation of a histogram from an image also takessignificantly more computational steps and memory space than extractionof an LDR histogram from an EDR histogram. Memory capacity of aprocessor is fairly limited, and the use of less memory space allows thespace to be used for other processes and purposes. Reduction ofprocessing time allows faster reaction time to capture a scene at itsoptimum exposure level. Hence this embodiment has the advantage ofcomputational efficiency over the other embodiments.

A third embodiment generates LDR histograms by generating a series ofLDR images with different exposures from an EDR image as shown in theblock diagram of FIG. 17. In this embodiment, similar to the previoustwo embodiments, an EDR image is generated. A first plurality of LDRimages of different exposures 402 is needed for the composition of theEDR image by the EDR image generation unit 404. A second plurality ofLDR images is generated by a plurality images generation unit 1702 and aplurality histograms calculation unit in 1704 to form LDR histograms 410from each image in the second plurality of LDR images. A tentativeexposure, Tx 412 is used by a selection of histograms unit 1706 toselect LDR histograms 410 that are provided by the system.

FIG. 18 further elaborates the process of this embodiment. As wasdiscussed with the previous two embodiments, a first plurality of LDRimages of different exposures are obtained in block 1802 and are used togenerate an EDR image in block 1804. In block 1806 a second plurality ofLDR images of different exposures are then generated from the EDR image.The exposures of these LDR images generated may or may not be the sameas the exposures of the LDR images obtained in block 1802. From eachimage, LDR histograms corresponding to the pixel intensities of LDRimages are calculated in block 1808. LDR histograms corresponding to anyparticular LDR image at a particular exposure, Tx are selected from theplurality of LDR histograms in block 1810. Re-selection of LDRhistograms can be done if the exposure value of the image correspondingto this set is deemed to be unsuitable by the optimum exposuredetermination unit 210 (see FIG. 2).

FIG. 19 shows four LDR images, LDR 1 1902, LDR 2 1904, LDR 3 1906, andLDR 4 1908, each with different exposures, that are used to generatecorresponding histograms in block 1806 of the embodiment shown in FIG.18. In this example, a number of histograms from different colorchannels are generated for each LDR image. For simplicity only thehistograms from the red channel are shown here out of the variouschannels available. Shown are Red 1 1910, Red 2 1912, Red 3 1914, andRed 4 1916, respectively belonging to LDR 1 1902, LDR 2 1904, LDR 31906, and LDR 4 1908.

FIG. 20 shows another embodiment which is similar to the embodimentshown in FIG. 17. LDR images of different exposure are obtained in block2002, which are then used to generate the EDR image in block 2004. Aplurality of LDR images of different exposures is then generated inblock 2006. The exposure values of the LDR images may or may not be thesame as the exposure values of the LDR images obtained in block 2002.Instead of creating histograms for all the images generated as describedin the third process embodiment, one LDR image is selected from theplurality of LDR images in block 2008. LDR histograms of this image arethen calculated in block 2010. Re-selection of the LDR image can be doneif the image is deemed to be of unsuitable exposure value by the optimumexposure determination unit 210 (see FIG. 2).

FIG. 21 shows another embodiment which is a slight variation of thesecond process embodiment. LDR images of varying exposures are obtainedin block 2102 and used to generate an EDR image in block 2104. EDRhistograms are then calculated in block 2106 using the pixel intensityof the EDR image. A plurality of LDR histograms analogous to differentexposure values are then generate from the EDR histogram in block 2108.In block 2110 LDR histograms of a tentative exposure value are thenselected. LDR histograms corresponding to a different exposure value canbe re-selected if the image corresponding to the tentative exposurevalue is deemed to be of unsuitable exposure value by the optimumexposure determination unit 210 (see FIG. 2). An LDR histogram having aspecific exposure value is defined as the same LDR histogram that willbe generated from an image of that specific exposure value.

FIG. 22 shows another embodiment. In this embodiment, no EDR image isneeded unlike the previous embodiments. In block 2202, a plurality ofLDR images of different exposures is obtained. Instead of using them tocompose an EDR image, LDR histograms are calculated from each of them inblock 2204. From this plurality of LDR histograms, an EDR histogram isthen generated in block 2206. The generation of an EDR histogram from aplurality of LDR histograms is similar to the generation of an EDR imagefrom a series of LDR images as described previously in FIGS. 6, 7, and8. LDR histograms of a tentative exposure value, Tx can be generatedfrom the EDR histogram in block 2210. Tx must be in a predeterminedrange: the minimum exposure to the maximum exposure value of theplurality of LDR images. A new Tx exposure value can be used if theimage of the scene with the current exposure value is deemed to be ofunsuitable exposure value by the optimum exposure determination unit 210(see FIG. 2). An LDR histogram having a specific exposure value isdefined as the same LDR histogram that will be generated from an imageof that specific exposure value.

FIG. 23 shows another embodiment, where an LDR image is obtained inblock 2302 from the image pickup apparatus 202 (see FIG. 2). LDRhistograms are subsequently generated from this image in block 2304. Ifthe optimum exposure determination unit 210 (see FIG. 2) determines theimage to be of unsuitable exposure, then another image can be taken fromthe image pickup apparatus 202 and the steps are repeated.

The histogram generation unit 206 in the imaging system 204 and itsvarious embodiments have been described. The histograms featuresextraction unit 208 or the features extraction unit 208 for short willbe described next.

A typical outdoor daylight picture usually comprises part of the sky, adominant subject, which is the picture's primary focus, and a brightobject that has to be adequately exposed. The subject may or may not bein highlight, depending on its surroundings. The presence of part of thesky in the picture can cause the auto-exposure of an imaging system tounderexpose the picture, causing the subject to be too dark. Hence, ifthe subject is the picture's focus, then there is a need to adequatelyexpose the image even in the presence of the sky. On the other hand, ifa bright object is present within the picture, focusing solely on thedominant subject that is not in highlight may increase exposure so muchso as to cause the bright object to be washed out. Therefore, there is aneed to strike a balance to achieve an exposure for both the dominantsubject and the bright object even in the presence of the sky in thepicture.

To achieve an adequately exposed image, the imaging system 204 includesa features extraction unit 208, shown in FIG. 2. The features extractionunit identifies the three features essential for an optimally exposedimage: the presence of the sky, a dominant subject, and a bright object.These features will be referred to as Sky-Peak, Significant-Peak, andRight-Peak, respectively.

As shown in FIG. 24, the features extraction unit 208 may include asky-peak detector unit 2404, a significant-peak detector unit 2406, anda right-peak detector unit 2408, respectively. The various detectorunits examine the image's histograms and extract specificcharacteristics.

These key features, Sky-Peak, Significant-Peak and Right-Peak provide aguide for the contents of histograms when an image has an optimumexposure. These three features are meant as a guide and are notrestrictive in nature. What comprises a peak in a histogram will bediscussed next to understand these features and what they represent inan image.

Peaks can be defined as a group of consecutive vertices in a histogramthat have only one local maxima. Three different types of peaks that canbe found in histograms are shown in FIGS. 25 a-c. The first type ofpeak, Peak 1 2502 is shown in FIG. 25 a. FIG. 25 a shows a portion ofthe histogram where a vertex of the portion is flanked on each side by avertex having a lower y-value. The highest point of the peak is referredto as the summit 2410, while the lowest point to the left of the peak isreferred to as the left tail of the peak 2508 and to the right, as theright tail of the peak 2512. FIG. 25 b shows a second type of peak thatis only found at the rightmost end of the histogram. Peak 2 2504 has asummit 2510 at the last bin. Peak 2504 has no right tail, only a lefttail 2508 denoting the lowest point of the peak. Finally Peak 3 2506 isshown in FIG. 25 c and is found only at the leftmost end of thehistograms. The summit 2510 of Peak 3 is at the first bin of thehistogram. Peak 2510 has no left tail but has a right tail 2512 denotingthe lowest point of the peak. The left and right tails of a peak mustalways be less than that of the summit. The vertices to the immediateleft and right of the tails must be higher than the tail itself. Thevertices to the immediate left and right of the summit must be lowerthan the summit itself. A common term used in the specifications is therightmost peak of any histogram. It actually refers to the peak with therightmost summit within the histogram.

Referring to FIG. 24 the features extraction unit 208 receiveshistograms 2402 and extracts features within the histograms. The threemain detectors 2404, 2406 and 2408 detect and measure the key features2410 of the histograms and these features will be used in the optimumexposure determination unit 210, by comparing the bin values of thesummit of the features to predetermined thresholds.

In outdoor daylight pictures, the presence of sky in part of the picturetypically causes the auto-exposure of a typical image device to make therest of the picture dark. The disclosed exposure control mitigates theinfluence of the sky region on the exposure control. The disclosedmethod first identifies the sky region, then subsequently eliminates orassigns a low-weighting factor to the histogram bins associated withpixels within the sky region in exposure control calculations.

The sky region of a daytime picture is usually bright. A firstcharacteristic of sky region is that it is typically made up of thebrighter pixels within the image. A second characteristic of the skyregion is that it is usually dominated by blue or green hues. Ifrepresented by the RGB color space, the sky region is usually dominatedby the G value and/or the B value of RGB. In the HSV color space, a peakshows up near the blue region of the hue channel, e.g. approximately 240degrees with a corresponding high V (brightness). In the YCrCb colorspace, bright blue is represented by a high positive Cb. Such detectionis not limited to the color space, but in this embodiment the RGB colorspace is used. This pair of characteristics translates to peaking at thefar right hand side of the green and blue histograms. A term, Sky-Peak,is given to such an observation and how to determine this will bedescribed in detail below.

The first component of the features extraction unit 208 is the sky-peakdetector unit 2404. FIG. 26 shows a process embodiment of the sky-peakdetector unit 2404. The sky-peak detector unit initially finds therightmost peaks in a set of three channel histograms (Red, Green andBlue) in block 2602. The rightmost peak of the blue histogram will thenbe compared against the green histogram in decision block 2604 to checkif the two peaks are similar to each other. A peak is said to besufficiently similar to another if the two of its three points: lefttail, summit or right tail, are within 1 bin apart. In addition, thepopulation in the similar points should be sufficiently close within adegree of tolerance. For example if peak A's left tail is at bin 3,summit is at bin 5 and right tail is at bin 8 and peak B's left tail isat bin 2, summit at bin 4 and right tail at bin 6, the population atleft tail and summit are then checked to see if it is within a tolerabledegree such as 10% from each other. If they are, then Peak A isconsidered similar to Peak B. If the blue rightmost peak is found to besimilar to the green rightmost peak, it is concluded that a sky existsin the image with its corresponding Sky-Peak within the histograms inblock 2614.

If the blue rightmost peak is found to be not similar to the greenrightmost peak, the rightmost peak of the red histogram will then bechecked with the rightmost peak of the blue histogram for similaritiesin decision block 2606. If the red rightmost peak is found to be similarto the blue histogram, the summit of the blue rightmost peak will thenbe compared against the red one to determine if the blue one is higherin decision block 2608. If the blue one is indeed higher, Sky-Peakexists. However, if either the red rightmost peak is not similar to theblue one or if they are similar but the blue summit is not higher thanthe red one then another comparison has to be made.

The third comparison checks if the summit of the rightmost peak of theblue histogram is higher than both the summits of the red rightmost peakand the green rightmost peak in decision block 2610. If the blue summitis higher, Sky-Peak exists. Otherwise a fourth comparison has to bemade. The sky-peak detector unit will check to determine if the lastvertex of the blue histogram is higher than the last vertices of the redand green histograms in decision block 2612. If the red and green lastvertices are lower than the blue last vertex, then Sky-Peak exists.However, if all criteria fail, it is concluded that there are noSky-Peaks in the histograms and consequently no sky in the image inblock 2616.

FIGS. 27 a-b, 28 a-b, 29 a-c and 30 a-c illustrate criteria used todetermine if a Sky-Peak exists. In FIGS. 27 a and FIG. 27 b, the firstcriterion of the comparison of the blue and green rightmost peak indecision block 2604 of the process shown in FIG. 26 is illustrated. FIG.27 a shows a green histogram 2702 of a particular image while FIG. 27 bshows a blue histogram 2706 of the same image. The green and bluerightmost peaks are highlighted using a dashed circle as shown in FIG.27 a as 2704 and in FIG. 27 b as 2708, respectively. The rightmost peaksof the two histograms are relatively similar as shown in FIG. 27 a andFIG. 27 b, which represents the presence of a Sky-Peak.

FIG. 28 a and FIG. 28 b show a red histogram 2802 and a blue histogram2808 of a particular image, respectively. The rightmost peak of the redhistogram 2804 in FIG. 28 a is compared against the blue rightmost peak2810 in FIG. 28 b and checked to determine if they are similar. Frominspection it can be seen that both peaks are not similar and the bluesummit 2812 is much higher than the red summit 2806 of the rightmostpeaks of the two histograms which suggest the existence of a Sky-Peak.

FIG. 29 a, FIG. 29 b and FIG. 29 c shows histograms used in decisionblock 2610 in the flowchart of FIG. 26. FIG. 29 a shows a red histogram2902 with a rightmost summit 2904, FIG. 29 b shows a green histogram2906 with a rightmost summit 2908, and FIG. 29 c shows a blue histogram2910 with a rightmost summit 2912. From visual inspection it can be seenthat the blue summit 2912 is the tallest which suggest the presence ofSky-Peak.

FIGS. 30 a-c show the histograms used in decision block 2612 offlowchart in FIG. 26. FIG. 30 a, FIG. 30 b, and FIG. 30 c, show a redhistogram 3002 with a last vertex 3004, a green histogram 3006 with alast vertex 3008 and a blue histogram 3010 with a last vertex 3012,respectively. As can be seen, the blue last vertex 3012 is higher thanboth the red last vertex 3004 and the green last vertex 3008. Therefore,Sky-Peak exists.

The features extraction unit 208 extracts a feature known asSignificant-Peak. Significant-Peak represents a dominant subject withinan image. For an image to be visually appealing this detail should notbe too bright or too dark. This feature can be used to determine if theimage is of suitable exposure. FIG. 31 shows a process for detecting aSignificant-Peak. The tallest peaks of the histograms of all fourchannels are first found in blocks 3102, 3104, 3106, and 3108. Thesepeaks must not correspond to the Sky-Peak, and are known asnon-Sky-Peak. From these four tallest non-Sky-Peaks from each of thehistograms, the peak with the rightmost summit will be selected in block3110 as a Significant-Peak 3112.

FIG. 32 provides a graphical example of the detection ofSignificant-Peak. A red histogram 3202 with its tallest non-Sky-Peak3204, a green histogram 3206 with its tallest non-Sky-Peak 3208, a bluehistogram 3210 with its tallest non-Sky-Peak 3212 and the luminancehistogram 3214 with its tallest non-Sky-Peak 3216 are shown. From visualinspection it can be seen that the red summit is at the last bin and isthe rightmost summit and the red peak 3204 is thus labeled aSignificant-Peak. For example, when an image is underexposed, thesubject of the focus is usually dark; thus the bin value of the summitof a Significant-Peak calculated from such a picture may be situated inthe darker bins.

The last feature that is to be extracted from the features extractionunit 208 is the Right-Peak. This can be an embodiment toSignificant-Peak or used on its own. The Right-Peak represents a brightobject within the image. An image with optimum exposure should have thisdetail neither too bright nor too dark. FIG. 33 shows how the right-peakdetector unit measures and detects Right-Peak from the histograms. Therightmost peak that does not correspond to the Sky-Peak is detected ineach of the four histograms in blocks 3302, 3304, 3306 and 3308. Whenall four rightmost non-Sky-Peaks are found, the peak with the tallestsummit is determined in block 3310 and labeled a Right-Peak in block3312.

FIG. 34 is a graphical representation of the process shown in FIG. 33. Ared histogram 3402, a green histogram 3406, a blue histogram 3410 and aluminance histogram 3414 of the same image are analyzed as shown on theleft hand side of FIG. 34. The rightmost peaks of each histogram that donot correspond to the Sky-Peak are shown as 3404, 3408, 3412 and 3416 ofthe red, green, blue and luminance histograms, respectively. The bluesummit of the peak 3412 is the tallest and is labeled the Right-Peak asshown on the right hand side of FIG. 34. For example, when an image isunderexposed, the subject of the focus is usually dark; such that thebin value of the summit of a Right-Peak calculated from such a picturemay be situated in the darker bins.

A visually appealing image should not be too dark nor too bright. Also,objects within the image, should have sufficient contrast. If an objectis washed out or too dark, there will be hardly any difference betweenthe object and its brightest and darkest point. Furthermore, detailswithin the picture will be lost when suboptimal exposure is applied tothe image.

To obtain a desirable picture four criteria can be used to determine ifan image has optimum exposure. First, the dark region in the pictureshould not occupy a large percentage of the entire image. Second, if thepicture contains part of the sky, there should be a sufficient contrastbetween the sky and the rest of the image. Third, the brightest subjector a significant detail within the picture that is not part of the skyshould have sufficient contrast between its brightest point and the restof the object. Last, details of objects in the picture should not bewashed out or too dark.

The image can be defined by three types of pixels: dark pixels,mid-range pixels and bright pixels. For example, in an 8-bit RGB image,dark pixels will most likely have RGB values below 50, bright pixelshave at least one of RGB values above 200, and the mid-range pixels willbe pixels with RGB values that fall in between. The mid-range pixels mayoverlap with some pixels belonging to the dark pixel range and thebright pixel range. In this embodiment, the three ranges are notstrictly exclusive and do overlap. In an optimally exposed image, thesky will consist of mostly bright pixels, while most objects willconsist mainly of mid-range pixels with a small proportion of the imagemade up of dark pixels.

FIG. 35 shows an embodiment for an optimum exposure determination unit210. Histograms 2402 of an image generated from the histogram generationunit 206 or other means, and features extracted from the histograms 2410from the feature extraction unit 208 or other means, are both providedto determine if the image is of adequate exposure.

The optimum exposure determination unit 210 includes a dark-detaildetermination unit 3502, a sky contrast determination unit 3504, apeak-contrast determination unit 3506, and a multitude of detailsdetecting unit referred to as a fluct determination unit 3508, whichmeasures the four criteria noted above. All four measurements determineif an image is of suitable exposure, and produce such an image 212.

The criteria will be referred to as the Dark Detail, Sky Contrast,Peak-Contrast and Fluct. The criteria are each first determined and thencompared to a predetermined threshold value for optimum exposuredetermination for histograms of each of the four channels. Themeasurements used to determine optimum exposure are:

-   -   (a) Dark Detail: Finding the proportion between the dark and        collectively the dark and mid-range pixels.    -   (b) Sky-Contrast: Examining the contrast between the dark part        of the sky and its dominant brightness if a sky exists.    -   (c) Peak-Contrast: Finding the contrast between the bright parts        of the significant detail and its dominant brightness in the        image.    -   (d) Fluct: Finding the presence of numerous details within the        picture.

Dark Detail measures the proportion between the dark pixels andcollectively the dark and mid-range pixels of an image. This proportionis used to determine if the image has suitable contrast. For example,there can be two ways to find Dark Detail, and the choice of the methodused depends on the existence of Sky-Peak in the histograms. FIG. 36shows an example of an 8-bin histogram being processed by a dark detaildetermination unit 3502. In block 3602 a first sum which calculates thepopulation of the first two vertices of a histogram is found. Thisconstitutes the dark pixels. The histogram is checked to determine thepresence of Sky-Peak in decision block 3604. If Sky-Peak is not present,the 8-bin histogram will then be divided into three parts. The first twovertices will be classified as the dark pixels, while the last 2 binswill be the bright pixels. The mid-range pixels will be the second andseventh bins. There is overlapping in the bins between the mid-rangepixels and the dark pixels as well as between the mid-range pixels andthe bright pixels. Other embodiments where the three parts do notoverlap can also be used. The mid-range pixels will be between the thirdand sixth bins. If Sky-peak is present, it is assumed that the brightpixels will be contained in the bins that represent the Sky-peak, whilethe first two vertices will still be assumed as the dark pixels.Mid-range pixels will be contained in the bin in between dark and brightpixels. In this embodiment, mid-range pixels will be contained in thebin between dark and bright pixels, including the last bin of the darkpixels and the first bin of the Sky-Peak. Other embodiments, themid-range pixels that do not overlap with the Sky-Peak can also be used.

To find the first criterion, Dark-Detail 3612, the ratio of the numberof dark pixels to that of both the dark and mid-range pixels are found.In the absence of Sky-peak, this is done by taking the ratio 3610 of sumof the population of the first two vertices performed in step 3602versus the sum of the population of the first sixth vertices determinedin step 3606. On the other hand, in the presence of Sky-peak, the upperbound of the mid-range pixels will shift from the sixth bin to the lefttail of the Sky-Peak in step 3608.

FIG. 37 a and FIG. 37 b show a histogram 3702 where there is no Sky-Peakpresent. A first sum of the population of the first two vertices 3704 ofan 8-bin histogram is found in FIG. 37 a which corresponds to block 3602of the flowchart in FIG. 36. A second sum of the population of the firstsix vertices 3606 is then calculated as shown in FIG. 37 b, per processblock 3606 of FIG. 36. The ratio of the first sum 3704 to second sum3706 represents Dark Detail, the contrast between the dark and allpixels of the image excluding the bright pixels.

FIG. 38 a and FIG. 38 b show a histogram 3802 with a Sky-Peak 3804. Afirst sum 3806 of the y-values of the first two vertices are calculated3806. However, the second sum 3808 in this method sums up the populationof the first vertex to the left tail of the Sky-Peak 3810 as shown inFIG. 38 b, per process block 3608 of FIG. 36. Dark Detail is determinedby finding the ratio of the first sum 3806 to the second sum 3808. DarkDetail is compared to a threshold value to determine if the exposure ofthe image being measured is adequate.

Sky-Contrast measures the contrast between the dark part of the sky andits dominant brightness if a sky exists. Sky-Contrast measures whetherdetails are lost when exposure increases. It is defined as the ratio ofthe left tail of the Sky-Peak to the summit, where the left tailrepresent the dark parts of the sky and the summit represent the binvalues of the pixels with dominant brightness of the sky. FIG. 39 showsa process performed by the sky-contrast determination unit 3504 tomeasure Sky Contrast. The histogram is first checked to determine if ithas Sky-Peak in decision block 3902. If Sky-Peak is present, then theratio of the y-value of the left tail of the Sky-Peak to the y-value ofthe summit is calculated in block 3904. This ratio is provided as aSky-Contrast measurement in block 3906. However if there is no Sky-Peakthen the Sky-Contrast cannot be determined as indicated by block 3908.

FIG. 40 a and FIG. 40 b show two histograms of two different images thateach contains a Sky-Peak. FIG. 40 a shows a histogram 4002 which has aSky-Peak with a summit 4006 at the last vertex and a left tail 4004.While FIG. 40 b shows a histogram 4008 which has a Sky-Peak with asummit 4008 and a left tail 4010. Sky-Contrast of each of the twohistograms is the ratio of 4004 to 4006 and 4010 to 4012, respectively.To determine if the current image is of suitable exposure, Sky Contrastmust be below a certain threshold value.

Peak-Contrast determines the contrast between the bright parts of asignificant detail and its dominant brightness in the image. It is usedfor optimum exposure measurement to insure the significant detailswithin an image do not get washed out as exposure increases. FIG. 41shows a process to determine Peak-Contrast. The histogram is checked todetermine if it contains a Sky-Peak in decision block 4102. If it does,the next rightmost peak that does not correspond to the Sky-Peakotherwise known as a non-Sky-Peak is found in block 4104. Otherwise ifthere is no Sky-Peak, the rightmost peak of the histogram is found inblock 4106. Next, the ratio of the population of the right tail of therightmost non-Sky-Peak found in either 4104 or 4106 to the population ofits summit is calculated in block 4108, and this is the measurement isprovided as a Peak-Contrast in block 4110.

FIGS. 42 a-b and 43 a-b illustrate a determination of Peak-Contrast. InFIG. 42 a and FIG. 42 b, the histogram 4202 is checked for a Sky-Peak. ASky-Peak is found to be present 4204 and the next rightmost non-Sky-Peak4206 is found. The Peak-Contrast is then found by taking the ratio ofthe population of the right tail of the rightmost non-Sky-Peak 4208 tothe population of the summit 4210. In the second histogram 4302 shown inFIGS. 43 a and 43 b, there is no presence of Sky-Peak. The rightmostpeak 4304 is found. The Peak-Contrast is the ratio of the population ofthe right tail of the peak 4304 to the population of the summit 4308.The Peak-Contrast must be less than a certain threshold in order for thecurrent image being determined to be of adequate exposure

Finally, the last measurement Fluct is needed to determine whether theexposure is adequate for a particular image. It measures the presence ofnumerous details within the picture which are represented in thehistograms as fluctuations or non-smoothness. Numerous details within apicture have to be kept within a certain optimum exposure. If the numberof details is too small, exposure may not be adequate. FIG. 44 shows aprocess for Fluct analysis. The absolute difference between thepopulation of a chosen vertex, Vertex A, and the average of itsimmediate left and right vertices are found in block 4402. This isrepeated in blocks 4404 and 4406 by changing the vertex in question(Vertex A). In this example the absolute difference for three verticesare found but any number of vertices can be used depending on the sizeof the bins and other factors. The sum of all the absolute differencesare then found in block 4408, and this is provided as Fluct in block4410.

For example, Fluct can be calculated as the sum of all the absolutedifferences between a vertex and the average of its immediate left andright vertex, for a series of consecutive vertices as shown in FIGS. 45a-d. FIGS. 45 a-d show a histogram 4502 with 8-bins. FIG. 45 a showsFluct1, which is the absolute difference between the y-value of vertex 34506 and the average y-values of vertex 2 4504 and vertex 4 4508. Fluct2is the absolute difference between the population of vertex 4 4508 andthe average of vertex 3 4506 and vertex 5 4510 and is shown in FIG. 45b. Fluct3 shown in FIG. 45 c is the absolute difference between they-value of vertex 5 4510 and the average of vertex 4 4508 and vertex 64512. In FIG. 45 d, Fluct4 is the absolute difference in y-valuesbetween vertex 6 4512 and the average of vertex 5 4510 and vertex 74514. Fluct is then the sum of Fluct1, Fluct2, Fluct3, and Fluct4. Fluctmust be above a certain threshold in order to determine the presence ofnumerous details within the picture, and hence the adequacy of exposure.This is one example of calculating the measurement Fluct. Other formulasmay be proposed to calculate Fluct as long as it indicates ripples inthe histogram curve.

The formula stated above can be expressed as:

$\sum\limits_{i}{{\left( {\frac{\left( {Y_{i - 1} + Y_{i + 1}} \right)}{2} - Y_{i}} \right)}.}$

Other formulas that may be proposed includes:

$\sqrt{\left( {\overset{n}{\sum\limits_{i}}\left( {Y_{i} - Y_{i - 1}} \right)^{2}} \right)}$$\sqrt{\left( {\overset{n}{\sum\limits_{i}}\left( {Y_{i} - \frac{Y_{i - 1}}{2} - \frac{Y_{i + 1}}{2}} \right)^{2}} \right)}$

Apart from the four parameters, Dark Detail, Sky-Contrast, Peak-Contrastand Fluct, as mentioned above, features extracted from the histogramscan also be used to determine the optimality of exposure. These featuresas mentioned in the embodiment of the feature extraction unit 208 arethe Sky-Peak, Significant-Peak and Right-Peak. The properties of thesethree features are compared against predetermined threshold values tocheck if the image is of optimum exposure.

Given these parameter and features, a slew of criteria can be used todetermine if exposure is sub-optimal. These criteria compare thefeatures and parameters with pre-determined threshold values. Thesethreshold values were experimentally found to give the best results withregards to the features and parameters. The parameters are usuallymeasured in terms of fractions or percentages except for Fluct, whichmeasures a non-smoothness in the histogram and features are used to findthe parameters. If Dark-Detail is greater than half, this means that thedark pixels occupy more than half of the image and the image is deemedtoo dark, and exposure is sub-optimal. If Sky-Contrast is greater than50%, there is a lack in contrast between the darkest part of the skywith the entire sky, which means that details of the image are lost dueto overexposure. Another sign of overexposure is when Peak-Contrastmeasures more than 50%.

On the other hand, exposure may be near optimal when there is a balancedratio for bright and dark pixels. This is achieved when Sky-Contrast isaround a fifth while Dark-Detail is around a third. Furthermore,exposure may be optimal when multiple details appear, which is indicatedby a high Fluct. The summit of Significant-Peak and Right-Peak will alsobe at the upper half of the histogram and Dark-Detail is less than afifth when exposure is near optimal.

The threshold values for the bin value for the summit ofSignificant-Peak or Right-Peak can vary between lower bin values tohigher bin values depending on whether the test is for an underexposedor overexposed image. Threshold values for Dark-Detail typically hasvalues less than 70% for an optimally exposed image while that forSky-Contrast varies between 5% to 60% depending on the whether the testis for under or overexposure and with what other parameters Sky-contrastis paired with. Threshold values for Fluct usually has high values whenmultiple details appear and these values are above 200 and thresholdvalues for Peak-Contrast is usually around 50%.

FIGS. 46 and 47 show an embodiment on how the criteria can be used todecide between increasing or decreasing exposure. FIG. 46 shows aprocess to determine whether to increase exposure. Decision block 4602checks to see if the bin value of the summit of Significant Peak orRight Peak is less than 3. If the test is positive, and if in decisionblock 4604 it is found that Dark Detail is greater than 55%, exposuremay be increased in block 4610. However, if the test from decision block4602 is negative and in decision block 4606 it is found that Dark Detailis greater than 45%, exposure may also be increased in block 4610. Indecision block 4608, the process determines if Dark Detail is greaterthan 35% and Sky-Contrast is less than 5%. If the test is positive,exposure may be increased as well, otherwise the exposure is notincreased.

FIG. 47 shows a process to determine whether to decrease exposure. Indecision block 4702 the process determines if Sky Contrast is greaterthan 50%, while decision block 4704 the process determines if SkyContrast is greater than 20% and Dark Detail is less than 35%. Indecision block 4706 the process determines if fluct is greater than 200.The process determines if Dark Detail is less than 35% in decision block4708 and in decision block 4710 it is determined whether the bin valueof the summit of Significant Peak or Right Peak is greater than 3 andDark Detail is less than 20%. Decision block 4712 the process determinesif the Peak Contrast is greater than 50% and Dark Detail is less than45% and also decides whether Sky Contrast is less than 10% and PeakContrast is greater than 50% in decision block 4714.

If either decision block 4702 or 4704 tests positive, exposure isdecreased in block 4716. Otherwise, if decision blocks 4706 and 4710both test positive, or if decision block 4706 tests negative whereasdecision block 4708 tests positive, then exposure is decreased as well.If all the above conditions fail, results from decision blocks 4712 and4714 are used. If either tests positive, exposure is decreased.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art.

1. A method for analyzing image, comprising: computing a histogram in adevice for an image of a scene, the image having a tentative exposurevalue; and, rejecting the tentative exposure value where a population ofa maximum in the histogram is less than a multiple of a population of aminimum in the histogram, wherein the minimum has higher intensity valuethan any other minimum in the histogram and the maximum is a maximumthat is nearest to the minimum and that has a lower intensity value thanthe minimum, wherein the histogram does not include pixels of skyregion(s).
 2. The method of claim 1, further comprising: selecting alesser tentative exposure value, wherein the population of the maximumis less than the multiple of the population of the minimum.
 3. Themethod of claim 2, wherein the histogram is generated from the image andthe image is generated by an image sensor.
 4. The method of claim 2,wherein the histogram is generated from the image and the image issynthesized from a plurality of images of the scene, the plurality ofimages having different exposure values, the tentative exposure valuebeing intermediate between a maximum exposure value and a minimumexposure value among the different exposure values.
 5. The method ofclaim 2, wherein the histogram is not generated from the image but issynthesized for the tentative exposure value from a plurality ofhistograms that are generated from a plurality of images of the scene,the plurality of images having different exposure values, the tentativeexposure value being intermediate between a maximum exposure value and aminimum exposure value among the different exposure values.
 6. Themethod of claim 1, further comprising: determining a ratio of thepopulation of the minimum to the population of the maximum; and,comparing the ratio with a threshold.
 7. The method of claim 6, whereinthe threshold is a predetermined percentage.